Characterization of Bipolaris species, their effects on switchgrass biomass yield and chemical components

Switchgrass (Panicum virgatum L.) is a promising biofuel crop; however, limited attention has been directed toward switchgrass pathogens and their impact on biomass yield. Bipolaris is one of the fungal pathogens that pose a potential threat to switchgrass production in the U.S. This research is focused on the impact of Bipolaris on switchgrass. The research objectives were 1) Assessment of isolates of four Bipolaris species for morphology and virulence diversity on switchgrass and impact on biomass yield, 2) Assessment of Bipolaris infection on switchgrass chemical components, 3) Assessment of switchgrass germplasm for disease resistance, and 4) Genetic characterization of Bipolaris species. Twenty-five representative isolates of Bipolaris oryzae, B. sorokiniana, B. spicifera, and B. victoriae varied in morphology and virulence on switchgrass. Bipolaris oryzae was the most virulent species and B. spicifera was the least. Low, intermediate and high virulence groups were identified across all isolates. Low-virulent isolates had limited negative impact on switchgrass whereas high-virulent isolates greatly impacted switchgrass, even at low inoculum density. High-virulent isolates should be targeted for disease resistance development in switchgrass. However, these virulence groups had similar effect on switchgrass chemical components. Bipolaris infection resulted in a significant increase in ash and lignin contents of switchgrass biomass, and a decrease in carbohydrate content, thereby reducing biomass fitness for conversion to drop-in fuels. Out of the nine switchgrass germplasms screened for disease resistance, six, namely Germplasm A, Germplasm B, Germplasm C, ‘Alamo’ (Bamert), ‘Trailblazer’, and ‘Summer’, were moderately resistant to B. spicifera (LA18) or B. oryzae (SK12), whereas none of the germplasms had resistance to B. sorokiniana (APCNR34) and B. victoriae (JA12). The moderately resistant germplasms can further be explored as resistance donors in breeding programs for development of cultivars with biofuel-valuable traits. Simple sequence repeats markers were developed from a small insert genomic library for B. sorokiniana. Sixteen polymorphic loci used to characterized fifteen B. sorokiniana isolates successfully cross-amplified at least one isolate of B. victoriae, B. spicifera and B. oryzae. These markers are valuable for genetic variability studies of Bipolaris species and a useful tool to formulate breeding strategies for development of resistant switchgrass cultivars.